Most Drug testing programs utilize the services of an independent physician called a Medical Review Officer (MRO) to review all test results. In the Case of a positive result, the MRO will normally contact the subject to determine if there is a medical explanation for the positive results. For example, eating poppy seeds before a test can result in a false positive for opiates. However, an MRO also knows that poppy seeds cannot cause certain levels of opiates, and certain additional testing can eliminate that.
There can also be tests that are "negative" but show an abnormal result, such as a "low creatine level", which can indicate an applicant attempted to dilute the sample by excessive drinking of water or some other form of alteration. That is also a result that a MRO would examine.
A good MRO has to be both a good detective and a skeptic. Many donors know a lot about drug testing and come up with stories, which, on the surface, sound plausible. The MRO has to carefully evaluate each explanation to determine whether the donor truly has a legitimate explanation or is trying to cover up illegal use of drugs.
The MRO or his/her staff reviews all negative test results. The MRO must review all confirmed positive tests. Under DOT regulations, the donor must be contacted by the MRO and given the opportunity to explain the positive test results.
It's the MRO's job to determine if the presence of the drug or its metabolite is indicative of drug abuse or legitimate drug use. The MRO has tools available to assist him/her in making this decision.
When there is legitimate use, the donor's physician is requested by the donor to provide the MRO with documentation substantiating the donor's claims of legitimate use.
The MRO serves a dual role. First, the MRO acts as a donor-advocate. The MRO questions the donor, trying to determine if the donor has a legitimate explanation for the presence of the drug or the metabolite in his/her urine. Second, the MRO protects the employer from wrongly hiring or firing a donor based upon an incorrect interpretation of a drug test result.